If your referring to the Paris gun, I believe it was one round every 45 min to an hour. Each round had to fired in exact sequence as barrel errosion on the previous shot made the bore slightly larger. Krupp steel was good, however autofretage had not been invented yet.

Krupps "Dicke Berta" (Big Bertha) (This is NOT ! the Paris Gun !)
http://www.waffenhq.de/panzer/dickeberta-01.jpg
A shot costet 1500 Marks back than, it could be used for 2000 shots. It was a strategic weapon, so the fiering rates were not so importend. I guess it took at least half an hour to reload, and the shots were all extremly planned, due to the high costs.
Range was up to 12 km with assisted ammo (Haubengranate) and 9 with normal high-explosive grens.
Paris gun: The Paris gun was a 21 cm Krupp gun which was able to shoot over 100 Km and was able to bombard Paris.

I can give you the fiering rates of a PZH2000, in my opinion the current best in the world: 8 shots in 42,3 seconds, 10 shots in 56,2 seconds. The rate is matched by some other artillery systems, but the PZH2000 is advanced in range and electronic targeting systems that can operate on their own and carry out strikes from differend Howizers on differend positions but hit the same target, in under 2 minutes from fiering and moving to another position.